1. Field of the Invention
The present invention relates to apparatus and methods for amplifying signals, especially apparatus and methods for linearly amplifying arbitrary information-modulated signals from low power levels (such as in the milliwatt range) to high power levels (such as in the kilowatt range). More specifically, the invention relates to linear amplification apparatus and methods including a modulator, especially a digital amplitude modulator, as an essential component.
2. Related Art
In U.S. Pat. No. 4,804,931, a digital amplitude modulator-transmitter is described. An amplitude modulator-transmitter embodying the teachings of the '931 patent is illustrated in FIG. 1.
The amplitude modulator 30 includes a predetermined number of quadrature hybrid power devices 32.sub.1 -32.sub.n. The quadrature hybrid power devices 32.sub.1 -32.sub.n are configured as combiners, and are arranged in cascade so that the output of one combiner is the first of two inputs of a successive combiner. In the exemplary illustration of FIG. 1, only four combiners are shown, for ease of understanding.
The construction and operation of a quadrature hybrid device is described in detail in the '931 patent, and need not be described here. However, for purposes of the present discussion, a quadrature hybrid combiner may be implemented as a four-port device having first and second inputs, one output, and one isolated port. The first and second inputs receive two signals of a given amplitude that are 90.degree. apart in phase. The input signals are combined to provide an output signal having an amplitude that is the sum of the amplitudes of the input signals. As shown in FIG. 1, ports 1 and 4 represent the first and second input ports. Port 3 represents an isolation port to which is attached a resistor representing a dummy load 34. Finally, port 2 defines the output port of each of the combiners.
An analog signal source 36, such as a microphone or a video generator, produces an analog signal that passes through an analog-to-digital converter (A/D converter, or ADC) 38. The digital output of the A/D converter appears on data lines 40.
As an example, there are four data lines, so that data consists of four-bit words. However, it is contemplated that 12- or 16-bit words may be employed to improve the quality of the signals being transmitted. Further, it is contemplated that a digital input signal may be fed directly to gates 42 without the need for A/D converter 38, should the modulating signal already exist in binary form.
Each of the bits, from the least significant bit (LSB) to the most significant bit (MSB), controls a respective gate 42.sub.1 -42.sub.n. Each of the gates is connected in series with a corresponding amplifier 44.sub.1 -44.sub.n. The series combinations receive an RF (radio frequency) signal from an RF signal generator 46, and provide respective second inputs to the combiners.
The output port of the LSB combiner 32.sub.1 represents the least significant bit of the digital word describing the instantaneous value of the signal to be modulated. The second input of the combiner 32.sub.2, adjacent the LSB combiner 32.sub.1, represents a value that is twice as significant as that of the second input of the LSB combiner 32.sub.1. Similarly, the second input of each successive combiner represents a value twice as significant as that input to the immediately preceding combiner.
Thus, the output of each combiner represents a binary weighted power signal. In particular, the signal appearing at the output of the MSB combiner 42.sub.n represents the sum of the power signals fed into the various combiners. This output signal is fed for transmission to a load, typically an antenna with conventional filtering.
The power efficiency of the digital amplitude modulator-transmitter is described in equation 24 of the '931 patent: ##EQU1## where m.sub.k is the modulation index (0=off, 1=on) of the k-th bit of the binary signal on path 40 and n is the number of bits in the word. Specifically, this is the power efficiency of the combining network with intentionally wasted power in the combiner reject loads when all RF driving sources are not gated ON (that is, when all m.sub.k are not unity). It is this intentional waste of power that causes the system in the '931 patent to be a linear amplitude modulator-transmitter.
The Applicant of the present patent application has realized that wasted power should be minimal at the highest capable peak power of the modulator-transmitter. In fact, when all driving sources (gates 42) ON (all m.sub.k =1), power efficiency is 100% in the combiner. However, combiner power efficiency cannot readily be improved if the modulation source is completely random, unpredictable and aperiodic. This is seldom the case, however, and it will be shown below with reference to a first aspect of the present invention, that significant power efficiency increases may be achieved when occurrences of peak power are predictable and frequent.
On a second matter relating to amplifiers in the '931 patent and amplifiers in general, it is desirable that output power be maintained constant, at least for a given time period. However, the magnitude of output power of RF amplifiers may drift for a variety of reasons, including variations in the temperature of the device, changes in power supply voltage, and load impedance. Despite these confounding influences, it is desirable from a modulation standpoint to maintain the output power of the amplifiers constant during periods in which they are turned on. A second aspect of the present invention is directed to an improvement which fulfills this need.
Third, it is desirable to minimize component count and circuit complexity, while serving as many of the above-mentioned needs as possible. A third aspect of the present invention fulfills this need.
A first aspect of the invention of U.S. Pat. No. 5,260,674 provides for a reduction in an otherwise maximum level of an analog input signal during a particular time period (for example, the time period of the high-amplitude synchronization pulse in an analog television format). In a particular preferred embodiment, the first aspect of the invention reduces the otherwise maximum value to a value which does not rise above the peak of a normally lower (for example, burst) level. When the normally high-amplitude portion is thus reduced in amplitude, efficiency is substantially increased when power efficiency is most important. To retrieve the full amplitude during the period of maximum amplitude (for example, the synchronization pulse period), an amplifier corresponding to the most significant bit of the analog signal being modulated, compensates for the reduced level of the input signal. In a particular preferred embodiment, the amplifier's compensation is accomplished by increasing the amplifier's supply voltage.
A second aspect of the invention of U.S. Pat. No. 5,260,674 provides an automatic gain control system especially suitable for the modulator-transmitter of the '931 patent. A preferred embodiment of an automatic gain control arrangement is based on a circuit that is governed by the output of a respective amplifier 44 as well as by the binary value of the respective bit corresponding to the amplifier and a reference value related to the bit's significance. The gain control port of the amplifier is preferably controlled by a filtered output of a comparator that compares the output of the amplifier to a suitable reference value. Applying this aspect of the present invention to the circuit disclosed in the '931 patent, the suitable reference value is specially related to the significance of the bit corresponding to the amplifier. This arrangement allows output power to be maintained substantially constant, providing improved modulation performance.
A third aspect of the invention of U.S. Pat. No. 5,260,674 minimizes component count by providing a hybrid modulator-transmitter. In the hybrid modulator-transmitter, lower-order bits of a signal to be modulated contribute to the modulated Waveform via an analog-processed portion which does not closely follow the teachings of the '931 patent. However, the more significant bits of the signal to be modulated are processed through a digitally processed portion, so that certain power-saving features of the invention are applied to the more significant portions of the digital words representing the signal to be modulated. This hybrid approach results in substantially equal power savings as a digitally processed circuit of greater overall complexity. In this manner, an optimum balance between power savings and component cost and complexity can be achieved.
Notwithstanding the significant advances reflected in the systems disclosed in U.S. Pat. No. 4,803,931, U.S. Pat. No. 5,367,272, U.S. Pat. No. 5,260,674 and U.S. Pat. No. 5,367,272 (Ser. No. 08/045,790), no one of those systems can provide efficient linear amplification of an arbitrary modulated input signal, especially from low power level (such as milliwatts) to high power level (such as kilowatts). Moreover, conventional linear amplifiers, as compared to modulators, have lacked the desirable features which can be provided by use of modulators based on the principles of U.S. Pat. No. 4,803,931.
Efficient linear amplification is particularly desirable in the field of television transmission, in which the modulated signals to be linearly amplified contain information which must be accurately preserved through the amplification process. In particular, accurate and efficient high power amplification of complex modulated signals, such as vestigial sideband (VSB) modulated signals, has not been achieved by known systems; known VSB amplification systems involve a substantial waste of power or do not possess the advantages of using modulators operating on the principles of U.S. Pat. No. 4,803,931.
In the modulators disclosed in U.S. Pat. No. 5,367,272 (Ser. No. 08/045,790), the summing which causes cancellation of a sideband is done at a high power level. This high power level summation wastes about half the signal's power because the signals input to the summation device are not coherent. Other systems in which signals such as VSB-modulated signals are generated at a high power level have relied on filtering the sidebands at the high power levels, also resulting in substantial power waste. Therefore, even if the digital amplitude modulator according to U.S. Pat. No. 4,804,931 were employed in the amplification process, the high power level sideband cancellation would be so wasteful of power as to offset the advantages of the digital amplitude modulator-transmitter.
Thus, there has long been a need to efficiently amplify arbitrary information-modulated signals to high power levels. For example, there has been a need in the an of analog television transmission, for apparatus and methods to efficiently amplify VSB-modulated signals, in which the partial cancellation of the sideband which is characteristic of VSB does not involve needless waste of power due to, for example, sideband cancellation or use of inefficient linear amplifiers.